I have a similar fluval tank and struggled with the same problem... Never did come up with a good solution. I tried the foam on the outlet as a diffuser and that helped a bit. Problem is that you need a big piece of foam (I used a 3" long section) an that detracts from the look of the tank. I put the foam on the inside of the line from the pump to the outlet... that only helped a bit. I figure that if you could build a plastic "muffler" with a wider outlet you would slow the flow (bernoulli's principle), but if you got the time/money to do that, you would be better of doing other things. In the end I angled the water towards a long piece of slate and used that to control what part of the tank was getting messed up by the flow (and to give my betta more quiet swimming space!).
Another heads up: If you get shrimp going in there, be prepared to fish for them in the filter section! Especially younger shrimp that get sucked in by the water flow alone.
Finally, one of the most amazing things to blow my mind in recent days:
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TOPIC: epiphyte's Fluval Spec 2g & Vase 2g - "Betta Homes"